1. 1 File Management Chapter 12

2. 2 File Management File management system consists of system utility programs that run as privileged (kernel) application For most applications  Input to applications is by means of a file  Output is saved in a file for long-term storage  Kbd and monitor can be regarded as “files”

3. 3 IO (see ch 11) In Unix-like systems all IO devices are made to look like files by the device driver  See WS chapter 11 for details

4. 4 File System Properties Long-term existence Sharable between processes Structure

5. 5 Terms Used with Files Field  Basic element of data  Contains a single value. EG avatar’s name or score  Characterized by its length and data type Record  Collection of related fields  Treated as a unit Example: player’s complete status record All the data related to a software object

6. 6 Terms Used with Files (cont) File  Collection of similar records  Treated as a single entity  Have file names  May restrict access Database (specialized variant of files)  Collection of related data  Relationships exist among elements

7. 7 Typical File Operations Create Delete Open Close Read Write

8. 8 Typical Operations (records) Retrieve_All Retrieve_One Retrieve_Next Retrieve_Previous Insert_One Delete_One Update_One Retrieve_Few

9. 9 File Management Systems The way a user of applications may access files Programmer does not need to develop file management software  cf Assembly: Programmer is responsible for where it is stored, how much space, initializing, deleting, recovering space etc.

10. 10 Objectives for a File Management System Meet the data management needs and requirements of the user Guarantee that the data in the file are valid Optimize performance Provide I/O support for a variety of storage device types

11. 11 Objectives for a File Management System (cont) Minimize or eliminate the potential for lost or damaged data  Locking, journaling etc. Provide a standardized set of I/O interface routines Provide I/O support for multiple users

12. 12 Minimal Set of Requirements User should be able to create, delete, read, write and modify own files User may have controlled access to other users’ files User may control what type of accesses are allowed to their own files User should be able to restructure own files in a form appropriate to the problem

13. 13 Minimal Set of Requirements User should be able to move data between files User should be able to back up and recover files in case of damage User should be able to access files by using symbolic names

14. 14 USB, CD, etc…

15. 15 Device Drivers Lowest level Communicates directly with peripheral devices Responsible for starting I/O operations on a device Processes the completion of an I/O request

16. 16 Basic File System Physical I/O Deals with exchanging blocks of data Concerned with the placement of blocks Concerned with buffering blocks in main memory

17. 17 Basic I/O Supervisor Responsible for file I/O initiation and termination Control structures are maintained Concerned with selection of the device on which file I/O is to be performed Concerned with scheduling access to optimize performance Part of the operating system

18. 18 Logical I/O Enables users and applications to access records Provides general-purpose record I/O capability Maintains basic data about file

19. 19 Access Method Reflect different file structures Different ways to access and process data

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21. 21 File Management Functions Identify and locate a selected file Use directory to describe location of all files plus attributes On a shared system, describe user access control Blocking for access to files Allocate files to free blocks Manage free storage for available blocks

22. 22 Criteria for File Organization Short access time  Needed when accessing a single record  Not needed for batch mode Ease of update  Not needed for read-only media CD-ROM, DVD, Kbd etc.

23. 23 Criteria for File Organization Economy of storage  Should be minimum redundancy in the data  Redundancy can be used to speed access such as an index Simple maintenance Reliability

24. 24 File Organization The Pile  Data are collected in the order they arrive  Purpose is to accumulate a mass of data and save it  Records may have different fields  No structure  Record access is by exhaustive search

25. 25 Pile

26. 26 File Organization The Sequential File  Fixed format used for records  Records are the same length  All fields the same (order and length)  Field names and lengths are attributes of the file  One field is the key field Uniquely identifies the record Records are stored in key sequence

27. 27 File Organization The Sequential File  New records are placed in a log file or transaction file  Batch update is performed to merge the log file with the master file

28. 28 Sequential File

29. 29 File Organization Indexed Sequential File  Index provides a lookup capability to quickly reach the vicinity of the desired record Contains key field and a pointer to the main file Index is searched to find highest key value that is equal to or precedes the desired key value Search continues in the main file at the location indicated by the pointer

30. 30 File Organization Comparison of sequential and indexed sequential  Example: a file contains 1 million records  On average 500,000 accesses are required to find a record in a sequential file  If an index contains 1000 entries, it will take on average 500 accesses to find the key, followed by 500 accesses in the main file. Now average is 1000 accesses

31. 31 File Organization Indexed Sequential File  New records are added to an overflow file  Record in main file that precedes it is updated to contain a pointer to the new record  The overflow is merged with the main file during a batch update  Multiple indexes for the same key field can be set up to increase efficiency

32. 32 Indexed Sequential File

33. 33 File Organization Indexed File  Uses multiple indexes for different key fields  May contain an exhaustive index that contains one entry for every record in the main file  May contain a partial index  Examples: Airline Reservations or Inventory Control

34. 34 Indexed File

35. 35 File Organization The Direct or Hashed File  Directly access a block at a known address  Key field required for each record  Features: rapid access, fixed length records, records accessed one at a time.  Examples: Directories, pricing tables, schedules, name lists

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37. 37 File Directories Contains information about files  Attributes  Location  Ownership Directory itself is a file owned by the OS Provides mapping between file names and files

38. 38 Simple Structure for a Directory List of entries, one for each file Sequential file with the name of the file serving as the key Provides no help in organizing the files Forces user to be careful not to use the same name for two different files

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42. 42 Two-level Scheme for a Directory One directory for each user and a master directory Master directory contains entry for each user  Provides address and access control information Each user directory is a simple list of files for that user Still provides no help in structuring collections of files

43. 43 Hierarchical, or Tree-Structured Directory Almost universally used Master directory with user directories underneath it Each user directory may have subdirectories and files as entries

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46. 46 Hierarchical, or Tree-Structured Directory Files can be located by following a path from the root, or master, directory down various branches  This is the pathname for the file Can have several files with the same file name as long as they have unique path names

47. 47 Hierarchical, or Tree-Structured Directory Current directory is the working directory Files are referenced relative to the working directory OS services such as link and shortcuts obscure, but do not change, the directory structure

48. 48 File Sharing In multiuser system, allow files to be shared among users Two issues  Access rights  Management of simultaneous access

49. 49 Access Rights None  User may not know of the existence of the file  User is not allowed to read the user directory that includes the file Knowledge  User can only determine that the file exists and who its owner is

50. 50 Access Rights Execution  The user can load and execute a program but cannot copy it Reading  The user can read the file for any purpose, including copying and execution Appending  The user can add data to the file but cannot modify or delete any of the file’s contents

51. 51 Access Rights Updating  The user can modify, deleted, and add to the file’s data. This includes creating the file, rewriting it, and removing all or part of the data Changing protection  User can change access rights granted to other users Deletion  User can delete the file

52. 52 Access Rights Owners  Has all rights previously listed  May grant rights to others using the following classes of users Specific user User groups All for public files What rights must the sysadmin have? – Alternatively what rights does she not need to have?

53. 53 Simultaneous Access User may lock entire file when it is to be updated User may lock the individual records during the update Mutual exclusion and deadlock are issues for shared access

54. 54 Fixed Blocking

55. 55 Variable Blocking: Spanned

56. 56 Variable Blocking Unspanned

57. 57 Secondary Storage Management Space must be allocated to files Must keep track of the space available for allocation

58. 58 Preallocation Need the maximum size for the file at the time of creation Difficult to reliably estimate the maximum potential size of the file Tend to overestimate file size so as not to run out of space

59. 59 Methods of File Allocation Contiguous allocation  Single set of blocks is allocated to a file at the time of creation  Only a single entry in the file allocation table Starting block and length of the file External fragmentation will occur  Need to perform compaction

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62. 62 Methods of File Allocation Chained allocation  Allocation on basis of individual block  Each block contains a pointer to the next block in the chain  Only single entry in the file allocation table Starting block and length of file No external fragmentation Best for sequential files No accommodation of the principle of locality

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65. 65 Methods of File Allocation Indexed allocation  File allocation table contains a separate one- level index for each file  The index has one entry for each portion allocated to the file  The file allocation table contains block number for the index

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68. 68 UNIX File Management Types of files  Regular, or ordinary  Directory  Special  Named pipes  Links  Symbolic links

69. 69 Inodes Index node Control structure that contains key information for a particular file

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73. 73 Linux Virtual File System Uniform file system interface to user processes Represents any conceivable file system’s general feature and behavior Assumes files are objects that share basic properties regardless of the target file system

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76. 76 Primary Objects in VFS Superblock object  Represents a specific mounted file system Inode object  Represents a specific file Dentry object  Represents a specific directory entry File object  Represents an open file associated with a process

77. 77 Windows File System Key features of NTFS  Recoverability  Security  Large disks and large files  Multiple data streams  General indexing facility

78. 78 NTFS Volume and File Structure Sector  The smallest physical storage unit on the disk Cluster  One or more contiguous sectors Volume  Logical partition on a disk

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